As those skilled in the art are aware, the maximum power output of a combustion turbine is achieved by heating the gas flowing through the combustion section to as high a temperature as is feasible. The hot gas, however, heats the various turbine components, such as the combustor, transition ducts, vanes and ring segments, which it passes when flowing through the turbine.
Accordingly, the ability to increase the combustion firing temperature is limited by the ability of the turbine components to withstand increased temperatures. Consequently, various cooling methods have been developed to cool turbine hot parts. These methods include open-loop air cooling techniques and closed-loop cooling systems. Both techniques, however, require significant design complexity, have considerable installation and operating costs and often carry attendant losses in turbine efficiency.
In addition, various ceramic insulation materials have been developed to improve the resistance of turbine critical components to increased temperatures. Thermal Barrier Coatings (TBC's) are commonly used to protect critical components from elevated temperatures to which the components are exposed.
The first stage of turbine vanes direct the combustion exhaust gases to the airfoil portions of the first row of rotating turbine blades and their corresponding ring segments. A ring segment is a stationary gas turbine component, located between the stationary vane segments at the tip of a rotating blade or airfoil. These ring segments are subjected to high velocity, high temperature gases under high pressure conditions. In addition, they are complex parts with large surface areas and, therefore, are difficult to cool to acceptable temperatures. Conventional state-of-the-art first row turbine vanes and ring segments may be fabricated from single crystal super-alloy castings, may include intricate cooling passages, and may be protected with thermal barrier coatings. Ceramic matrix composites (CMC) have higher temperature capabilities than metal alloys. By utilizing such materials, cooling air can be reduced, which has a direct impact on engine performance, emissions control, and operating economics.
One of the limitations of CMC materials, whether oxide or non-oxide based, is that their strength properties are not uniform in all directions (e.g., the inter-laminar tensile strength is less than 5 percent of the in-plane strength). Anisotropic shrinkage of matrix fibers results in de-lamination defects in small radius corners and tightly curved sections, further reducing the already low inter-laminar properties. Thus, the use of CMC materials for gas turbine components has been limited.